the elizabethan collars were originally devised to stop the animal from chewing through surgical stitches if they were operated in area that are accessible (neutered / spayed, legs operations et al), so they are oversized as standard. as the wound is in the neck, I assume is very close to the base of the collar, so you can trim away a fair bit as long as she can't reach the wound and scratch it with her paws.

probably a good idea to get hold of the vet and ask his/her opinion